Throughfall spatial patterns translate into spatial patterns of soil moisture dynamics - empirical evidence

Throughfall heterogeneity induced by the redistributionof precipitation in vegetation canopies has repeatedly been hypothesized toaffect the variation in the soil water content and runoff behavior, especiallyin forests. However, observational studies relating the spatial variation in thesoil water content directly to net precipitation are rare, and few confirmmodeling hypotheses. Here, we investigate whether throughfall patternsaffect the spatial heterogeneity in the soil water response in the main rootingzone. We assessed rainfall, throughfall and soil water content (at two depths,7.5 and 27.5 cm) on a 1 ha temperate mixed-beech forest plot in Germanyduring the 2015 and 2016 growing seasons using independent, high-resolution,stratified, random designs. Because the throughfall and soil water content cannotbe measured at the same location, we used kriging to derive the throughfallvalues at the locations where the soil water content was measured. We firstexplored the spatial variation and temporal stability of throughfall and soilwater patterns and subsequently evaluated the effects of input (throughfall), soilproperties (field capacity and macroporosity), and vegetation parameters(canopy cover and distance to the next tree) on the soil water content anddynamics. Throughfall spatial patterns were related to canopy density. Althoughspatial autocorrelation decreased with increasing event sizes, temporallystable throughfall patterns emerged, leading to reoccurring higher- and lower-input locations across precipitation events. Linear mixed-effects modelanalysis showed that soil water content patterns were poorly related tospatial patterns of throughfall and that they were more influenced by unidentified,but time constant, factors. Instead of the soil water content itself, the patterns of its increase afterrainfall corresponded more closely to throughfall patterns: morewater was stored in the soil in areas where throughfall was elevated. Furthermore, soil moisture patterns themselves affected the local soil water response, probably by mediating fast drainage and runoff. Locations with a lowtopsoil water content tended to store less of the input water, indicatingpreferential flow. In contrast, locations with a high water contentstored less water in the subsoil. Moreover, the distance to the next tree and macroporositymodified how much water was retained in soil storage. Overall, throughfallpatterns imprinted less on the soil water content and more on the soil waterdynamics shortly after rainfall events; therefore, percolation rather than thesoil water content may depend on the small-scale spatial heterogeneity in canopyinput patterns.